Abstract Attention-deficit/hyperactivity disorder (ADHD) is associated with widespread peer difficulties that predict long- term negative outcomes including substance use and depression. The majority of research has focused on boys with the disorder, although there is accumulating evidence that girls with ADHD display relatively broader and more severe peer dysfunction. Relatively little is known about the neural mechanisms underlying the peer dysfunction of girls with ADHD, hindering progress in developing targeted intervention strategies to improve peer functioning. Using a novel and realistic laboratory peer interaction task (Island Getaway), we recently identified alterations in neurophysiological processing (i.e., event-related potentials; ERPs) of both peer rejection and acceptance feedback that were associated with ADHD symptoms in a community sample of young adolescents. Specifically, ADHD symptoms were associated with an enhanced early attentional ERP to peer rejection (N1) and blunted later ERP to peer acceptance (reward positivity [RewP]). These findings suggest two distinct pathways contributing to peer dysfunction among youth with ADHD. Enhanced attention to rejection cues may lead to increased hostility with peers, while reduced responsiveness to acceptance cues may contribute to difficulties establishing and maintaining positive friendships. The proposed work extends these intriguing preliminary findings by examining brain-behavior associations between neurophysiological processing of peer rejection and acceptance cues and peer difficulties measured across self-report and behavioral observation in girls. Adolescent girls (ages 11-14) with (n=40) and without (n=40) ADHD will complete the Island Getaway task to assess neurophysiological processing of peer rejection and acceptance cues. Self-reported hostile and prosocial behavior will be assessed using ecological momentary assessment (EMA), and a laboratory-based peer interaction task will be used to assess observed hostile and prosocial behavior. Enhanced ERPs to peer rejection cues will be examined as a mechanism linking ADHD and hostile behavior towards peers. Blunted ERPs to peer acceptance cues will be examined as a mechanism linking ADHD and low prosocial behavior with peers. This exploratory study will inform understanding of brain-behavior mechanisms of peer difficulties in girls with ADHD and integrate data across levels of analysis to test specific pathways to peer problems.